1. Field of the Invention
The present invention relates to optical communication.
2. Related Art
Optical communication is in increasing demand. In optical communication, digital data is carried in an optical signal. The digital data consists of bits of information (e.g., “1s” and “0s”). Such data is transmitted in an optical signal using modulation. Examples of modulation which can be used in optical communication are modulation based on amplitude, frequency, phase, polarization, or a combination thereof.
Communication between a source node and destination node is conducted through one or more links. A link can include logical and/or physical layers to support data transfer. An optical wireless communication system includes a physical layer with an optical wireless link. An optical wireless link can be any optical link through a turbulent medium such as, the atmosphere. For example, an optical wireless link can include, but is not limited to, an optical line of sight (LOS) link that uses a transmitter, such as, a laser or light-emitting diode (LED). Data is transmitted on the emitted beam using some form of amplitude, frequency, phase, or polarization modulation, or a combination thereof.
Optical wireless links are increasingly popular as they can provide relatively low cost, high performance communications over short distances—typically up to a few kilometers at sea level. Other advantages of optical wireless links are: no FCC spectrum allocation is required for their use, they have a high degree of physical security because of the directionality of the transmitted beam, the communication channel infrastructure exists with a low probability of detection, and easy set-up and removal. Optical wireless links have a further advantage in that no communication channel infrastructure such as, optical fibers, needs to be installed. Of course, an optical wireless link can be a separate stand alone path between a source node and destination node or it can be combined with any other type of conventional communication link, such as, a fiber link, RF link, or wired link.
In an optical wireless link, the optical beam passes from transmitter to receiver along a path through the atmosphere. Since the atmosphere is a turbulent medium the optical wavefronts are distorted in passing from transmitter to receiver. Such fluctuations in received power are unpredictable and can result in the loss of data (that is, “1” bits can be detected as “0” bits). This fluctuation problem is known as fading. Because of the potential for fading, there is some probability that the fluctuating received intensity will fall below a threshold value and lead to unacceptable bit error rates on an optical wireless link.
Different approaches can be used to reduce fading. One approach is to simply re-send packets of data. In this case, if data does not arrive satisfactorily, then it is re-transmitted. Unfortunately, this scheme may significantly reduce the overall data transfer rate and increase system complexity. Other approaches include the following:
(1) Increasing the transmitter power, which is not always possible because of electrical power requirements and eye-safety issues;
(2) Increasing the effective size of the receiver, which may not always be possible because fast optical receivers are generally of small size, and keeping light collected over an aperture (lens, mirror, or non-imaging light collector) focused on a small detector may require active tracking, or over-filling of the detector;
(3) Introducing spatial diversity, which amounts to sending several optical beams from transmitter to receiver along generally parallel paths, which are hopefully uncorrelated with regard to turbulent effects in the beams traveling along each path; and
(4) Introducing phase scrambling at the transmitter, which can reduce the degree of scintillation at the receiver.
These approaches, however, have limitations such as complexity or additional cost. Also, despite all these approaches, at some level fading will remain.
What is needed is a method and system for optical wireless link communication that reduces fading.